1. Technical Field of the Invention
The invention relates to a magnetic accelerometer and, more particularly, to such an accelerometer that is used to detect excessive vibrations, for example, as occur as a result of an unbalanced load in a washing machine.
2. Description of the Related Art
In electromechanical devices, it is sometimes necessary to sense operational conditions such as undesirable excessive vibrations which could cause a malfunction. For example, washing machines have an internal tub that is designed to spin at very high speeds when loaded with wet clothing. Occasionally, the laundry in the tub may clump together and, therefore, provide an unbalanced load. The unbalanced load can cause the tub to rock back and forth as it rotates. The severe vibrations resulting from this rocking movement can damage the washing machine if they are allowed to continue.
Accordingly, washing machines have been equipped with sensors that detect such undesirable rocking motions or vibrations and either stop the spinning of the tub or signal an unbalanced load. The load can then be redistributed and the spinning cycle can resume.
Electromechanical accelerometers have been used in washing machines to detect an unbalanced load by sensing the sharp accelerations and decelerations of the spinning tub as it rocks back and forth. Accelerometers for this application have typically used a plastic or metal slug that is moved against the return force of a spring in response to the sharp acceleration of the rocking tub. The return force of the spring can be adjusted to accommodate movement of the slug in response to a predefined magnitude and frequency of lateral movements of the tub. Vibrations of sufficient frequency and intensity therefore cause the slug to move along a predetermined path against the force of the return spring until the slug strikes and activates a switch. The activated switch can generate an audible alarm, for example from a buzzer, or a visual alarm, for example from a lighted bulb. When the excessive vibration stops, the slug is automatically returned to its initial rest position by the force of the spring.
Known vibration accelerometers have tended to be relatively expensive and have not allowed a wide range of return force adjustments for the spring. Also, the return spring can lose its resiliency over time and therefore change the amount of lateral vibration force required to trigger an alarm condition. It would, therefore, be desirable to provide an improved, relatively low cost accelerometer that has relatively well-behaved dynamic properties and that defines an alarm trip-point that is readily adjustable over a relatively wide range and is repeatable and predictable over time.